Two multi‐curie samples of 40‐hr. La140 were irradiated with slow neutrons at the Los Alamos homogeneous pile for 24 hours. The small amount of 3.7‐hr. La141 formed was allowed to decay to 28‐day Ce141 which was then radio‐chemically extracted and counted. From the activity measured, the neutron flux, and the known half‐lives, a value of 3.1±1.0 barns was calculated for the cross section of 40‐hr. La140. The thermal neutron absorption cross sections of stable Ce140 and Ce142 were also measured and found to be 0.27±0.06 and 0.105±0.020 barn, respectively, per atom of the naturally occurring element.

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